This invention relates to the field of agricultural devices, and more particularly to polymeric or plastic materials used in temporary or permanent agricultural constructions, such as hotbeds and greenhouses. The agricultural plastic sheeting or films have specific technical properties designed for various growing conditions and are used for cladding greenhouses, hotbeds, tunnels, soil-mulching, etc. These plastic materials are made from high and low pressure polyethylene and polyvinyl chloride resins and manufactured as 20 to 300 .mu.m thick films by numerous companies. Such films in quantities of thousands of tons are used to cover agricultural plants in trenches, to protect seeding in hotbeds, to cultivate crops and flowers in greenhouses and to perform experiments in greenhouses. Re-covering of said constructions with the film materials allows one to adapt temporary covered-soil constructions for different crops every season--if needed--thus controlling both gaseous exchange in plants and illumination conditions, with substantial economic savings. Furthermore, the polymeric covering material is three to four times cheaper than silicate glass.
One can incorporate into the polyethylene films UV or light stabilizers to expand the service life of these films [1]. One can also add anti-misting additives in order to prevent plant foliage and fruit scorching, which occurs from the condensation of water droplets. These additives also improve the quality of the crops. Other additives are designed to provide internal temperature variations in the greenhouses beneficial for a particular growth season effecting the yield of the crops.
In recent years, organic additives were used to yield colored films. The pigments and polychromatic additives based on these materials exhibit color when exposed to solar light or ultra-violet radiation. The rationale for incorporating these types of additives is that it allows a more efficient use of solar radiation since the coloration of the film corresponds to the photosynthetic active radiation (PAR)--the solar light fraction useful in producing plant growth.
It is well known that plants through their development stages react differently to the intensity and wavelength of the light. The band of 400 to 700 nm represents the PAR [2]. The absorption of the PAR radiation depends on absorbing species such as chlorophyll, carotene, etc., which have characteristic absorption wavelengths in that region. The plant's photosynthetic activity responds to specific wavelengths of the light [3]. In recent years, there has been particular interest in the active photosynthetic radiation over the range of 450 to 600 nm for the control of morphogenic, physiological and molecular development with regard to productivity of plant crops such as flowering, growth, yield and quality of produce [4-6]. Additional development of plastic materials using photosensitive additives was reported for the selective application of PAR [7], increasing the intensity of reproduction processes in green plants through generation of additional red-orange light.
The action mechanism of such polymeric materials with additives involves biophysical phenomenon of acceleration of photosynthesis in the chlorophyll pigment through increased absorption of red or orange light that results in additional absorption of carbonic acid from air, synthesis of carbohydrate, and release of oxygen. Red-orange light complementary to sunlight is excited in the additive under the action of ultraviolet rays (UV), a fraction of which in sunlight is 3-6%--it is believed that solar UV radiation is not absorbed by green plants. To transform unused UV solar energy into red-orange light useful for plants, authors of the work [7] suggested to add complex organic additives such as adonifen or europium phenanthrolinate, the photo-transforming materials, with a concentration of 0.05-1% in the film polymeric material. By using these photo-transforming additives, the yield of different crops was increased--through so-called photo-reproduction effect--as it was observed previously in temporary or permanent greenhouses when red light-emitting fluorescent lamps were used [7].
However, observations for a number of years showed numerous drawbacks of the first-generation polymeric photo-transforming material:
A photo-transforming effect lasted no more than 1-3 weeks; PA1 Non-reproducibility of the photo-reproduction effect for plants of different kinds and groups, e.g. increase in lettuce crop was not repeated for tomato or carrot crops, etc.; PA1 high cost of the film material because of extremely high cost of the organic photo-transforming additive; PA1 low stability in different climactic zones and low mechanical strength of the polymeric material (trade-mark "Polysvetan"), resulting in fast degradation and destruction of the material during several weeks; PA1 high toxicity and environmental hazard associated with using the polymeric material containing these additives. PA1 The concentration of additives is too high, significantly increasing the costs. PA1 The mechanical strength reported was reduced and in many cases was less than in the control film. PA1 The films exhibited an early aging in less than a year of service. PA1 Optical properties of these photo-transforming films were not studied therefore preventing optimization of the transmitted light spectrum. PA1 reasonably high brightness of red-orange emission, PA1 steady increase in yield of crops transplanted in covered soil, PA1 ecological safety and stability in different climactic zones, PA1 moderate increase in the cost of the polymer sheeting,
The use of photo-transforming organic additives thus resulted in inferior mechanical and life properties of the plastic films with poor durability and maintenance due to the premature degradation [7].
Another noticeable effort in the field of photo-transforming additives by a group from the Applied Chemistry Center (ACC) at Saltitlo, Coahuila, Mexico, was granted a Mexican patent [8] for greenhouse film that makes the PAR more effective and thereby gives an increase in crop yield and quality--specifically, in lettuce. In a subsequent paper by Lozano et al [9], a 40% improvement yield of the lettuce crop was reported. Here, again, several shortcomings of the so-called polychromatic additive plastic films developed by the ACC group were reported [9]. In our opinion, the following shortcomings prevent their films from practical applications in the US:
Meanwhile, the essential drawbacks of the first-generation covering photo-transforming materials [7] stimulated the development of a new material in which the organic complex photo-transforming additive was fully or partially substituted by inorganic photo-transforming materials based on Oxysulfides or other oxyanionic materials [10]. In accordance with the Russian patent [10], 0.01-0.5 weight percent inorganic photo-transforming material was added to the plastic resin. After fabrication of sheeting, it was uniformly distributed in the plastic film. This sheeting generated additional red-orange light useful for growth of the vegetables, fruits, crops, flowers, tree cuttings and saplings, and other green plants. This plastic film--trade name "Redlight"--was widely used in Russia and contiguous countries from 1994 to 1998. At present, this plastic film represents a modern technological level of practical polymeric sheeting for agricultural applications mentioned above. The following positive features of "Redlight" polymer sheeting:
do not eliminate a substantial drawback of the material: a significant rapid decrease of its physical and mechanical parameters within one summer season. For instance, the tensile strength of 6-mil--150 micron--thick sheeting decreased from 200 kg/cm.sup.2 initially to 60 to 80 kg/cm.sup.2 in two months only. Such lowering of the tensile strength resulted in fully destroyed, non-functional sheeting. One needs to re-cover greenhouses and hothouses with "Redlight" sheeting several times--up to three--during the agricultural season thereby increasing the costs to unpractical levels. Until now, numerous unsuccessful attempts to pinpoint the main reasons and eliminate the dramatic lowering of physical-mechanical properties of the covering film with inorganic photo-transforming additives were undertaken causing a decrease in the consumption of the "Redlight" sheeting. The issues of the film's durability and maintenance that translate into the film's lifetime and practical cost have to be qualitatively resolved in order to make the film's applications practical.